Neither party appeared to have a clear advantage in the battle for the House of Representatives, which Republicans now control by a narrow margin.
But Tuesday's results ensured Republicans would be able to help Trump appoint conservative judges and other government personnel if he wins the presidential race, or block much of Democrat Kamala Harris's agenda if she prevails.
Governor Jim Justice is projected to win an open Senate seat in West Virginia. (AP PHOTO)
Republican Jim Justice was projected to win an open Senate seat in West Virginia shortly after polls closed, taking over the seat previously held by Joe Manchin, a Democrat-turned-independent.
In Ohio, multiple US media outlets projected that Republican Bernie Moreno would defeat incumbent Democrat Sherrod Brown.
Those two victories ensured Republicans will hold at least a 51-49 majority in the Senate, with further gains possible as results in other competitive races come in.
Republicans also posted gains as they sought to retain control of the House, which they currently control by a narrow 220-212 majority.
They picked up three seats from Democrats in North Carolina, where they had redrawn district lines to bake in their advantage, while Democrats won control of a Republican-held seat in Alabama that had been redrawn to comply with a US Supreme Court order to create a Black majority district.
Democrats now need to flip at least six seats to take control of the 435-seat chamber.
Congress will have its first openly transgender member after Sarah McBride was elected to the House. (AP PHOTO)
Voters made history in Delaware, where they elected Democrat Sarah McBride as the first openly transgender member of Congress.
As in the presidential election, the outcome will likely be determined by a small slice of voters.
Fewer than 40 House races are seen as truly competitive.
Republicans stand a chance to widen their Senate majority further if they win in Montana, where Democrat Jon Tester faces a tough re-election battle, and prevail in several competitive Midwestern states.
But they are unlikely to end up with the 60-vote majority needed to advance most legislation in the chamber.
In Texas, incumbent Republican Ted Cruz was projected to win re-election, holding off Democrat Colin Allred.
Democrats could easily pick up enough seats to win control of the House, analysts say. (AP PHOTO)
In Nebraska, Republican Senator Deb Fischer was holding off a surprisingly strong challenge from an independent candidate, Dan Osborn, who has not said whether he would line up with Democrats in the Senate if he were to win.
The Senate was set to include two Black women serving simultaneously for the first time, as several media outlets projected Democrat Angela Alsobrooks would win in Maryland, and Democrat Lisa Blunt Rochester won in Delaware.
The outcome was still up in the air in the House, despite Republicans' gains in North Carolina.
Analysts say Democrats could easily pick up enough seats to win control of the chamber, though there are no signs of a "wave" election, akin to 2018 or 2010, that would result in a decisive power shift.
With at least 200 seats safe for each party, the winning side will likely end up with a narrow majority that could make governing difficult.
That has been evident in the past two years as Republican infighting has led to failed votes and leadership turmoil and undercut the party's efforts to cut spending and tighten immigration.
Tight races in the heavily Democratic states of New York and California could determine House control, though the outcome might not be known for several days as California typically takes several days to count its ballots.
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